A neurologist testified Friday at the trial of an American student accused of killing her British roommate that stress could have affected her memory during questioning, news reports and a lawyer said.

The expert was called by the defense team of Amanda Knox, who is on trial with her former boyfriend Raffaele Sollecito for the 2007 slaying of Meredith Kercher, 21. They deny wrongdoing.

In the days after the slaying, Knox gave authorities conflicting statements, at one point saying she was in the house the night Kercher was stabbed to death in her bedroom.

Knox and Sollecito have said they smoked hashish the night of the killing.

Neurologist Carlo Caltagirone told the court in the city of Perugia that Knox was under stress after long police questioning, which might have confused her, said lawyer Francesco Maresca, who represents Kercher's family.

"To be questioned for long hours in a foreign country without fully realizing the situation one is in ... can lead to a lot of stress," Caltagirone was quoted as saying by the ANSA news agency.

Knox initially fingered as the killer Diya "Patrick" Lumumba, a Congolese man who owns a pub in Perugia where she worked. Because of her accusation, Lumumba was briefly jailed. He was later cleared and is seeking defamation damages from Knox.

Knox, 22, of Seattle, Washington, has since maintained that she spent the night of the murder at Sollecito's house elsewhere in Perugia.

In June, the American testified that she was beaten by police and confused when she was questioned. She said it was the pressure that led her to accuse Lumumba.

Police have denied any misconduct.

Lumumba, who was in court Friday, told reporters that Knox came up with his name to "sidetrack the investigation," according to ANSA.

A third defendant, Rudy Hermann Guede of Ivory Coast, was arrested and convicted in a separate trial last year. He was sentenced to 30 years in prison. He denies wrongdoing and has appealed his conviction.

Also on Friday, another defense witness, forensic expert Walter Patumi, testified that a knife that prosecutors say could be the murder weapon is not compatible with Kercher's wounds.

Prosecutors say the knife — with a 6 1/2-inch blade — had Kercher's DNA on the blade and Knox's on the handle. It was found at Sollecito's house.

Defense lawyers maintain the knife is too big to match Kercher's wounds and the amount of what prosecutors say is Kercher's DNA is too low to be attributed with certainty.

The trial began in January. Closing arguments are expected to begin in October, lawyers say.

The court also might request an independent review of the scientific evidence, delaying a decision.